Flat cables comprising parallel fused portions and twisted pair portions provided alternately with the parallel fused portions are known in the art. In these flat cables, the parallel fused portions each are composed of separately insulated conductors which have been disposed side by side, adjacent insulated conductors in their respective insulative layers being fused to each other to form the parallel fused portion. The parallel fused portions permit simultaneous connection of conductors in the flat cable to a connector. On the other hand, the twisted pair portions extend from the parallel fused portions and each are composed of a plurality of twisted pairs juxtaposed to each other or one another. The plurality of twisted pairs each are composed of two conductors which have been separately insulated by an insulative layer and have been twisted together. The twisted pair portions are provided in order to improve electric characteristics of the flat cable.
Conventional processes for producing these flat cables will be explained. According to one conventional process for producing a flat cable (hereinafter often referred to as "first conventional production process"), a plurality of insulated conductors are fed into a twister where adjacent insulated conductors are twisted together to form twisted pairs. The twister is provided with twist ports that are arranged in a row.
When the insulated conductors in their portion corresponding to the twisted pair portion are passed through the twist ports, the twists ports are rotated to form a twisted pair portion. On the other hand, when the insulated conductors in their portion corresponding to the parallel portion are passed through the twist ports, the rotation of the twist ports is stopped to permit the insulated conductors to be parallel aligned to form a parallel aligned portion.
The front end of the parallel aligned portion delivered from the twist ports is first held by forks with one of them providing a space, and the rear end of the parallel aligned portion is then held with the other providing a space, thereby preventing the twists in the twisted pair portion from adversely affecting the parallel aligned portion.
The width of the twisted pair portion and the width of the parallel aligned portion gradually decrease with the progress of the work. This is achieved by virtue of holding by the forks while leaving a slight space. The insulated conductors are then fed into a mold comprising a combination of two molds, an upper mold and a lower mold, for forming a parallel fused portion.
When the forks holding the parallel aligned portion respectively reach both sides of the mold, the upper and lower molds are closed and the mold is then moved in synchronization with the insulated conductors. This permits adjacent insulated conductors constituting the parallel aligned portion to be heat fused to each other to form a parallel fused portion.
After the upper and lower molds are closed, the forks leave the line and are used again. The forks are mounted on a rotary belt that returns the forks to a position at which the twister is provided.
Upon the completion of the formation of the parallel fused portion in the mold, the upper and lower molds are opened to release the formed parallel fused portion, followed by re-operation of the twister. The above procedure is repeated to produce a predetermined flat cable.
Another conventional process for producing a flat cable (hereinafter often referred to as "second conventional production process) will be explained.
In the first conventional production process, the twists ports in the twister are arranged in a row. On the other hand, in the second conventional production process, the twist ports are arranged in the vertical direction as well as in the lateral direction.
Specifically, in the twister, the twist ports are arranged in both the vertical direction and the lateral direction in such a manner that the pitch between insulated conductors passed therethrough is identical.
In the second conventional production process, instead of the forks used in the first conventional production process, an aligning device is used for aligning the insulated conductors. The aligning device comprises a pair of devices, and fine separators are provided between the pair of devices. Each of the fine separators is inserted between adjacent two insulated conductors constituting the insulated conductor pair. The devices are disposed respectively at both ends of the parallel portion. In this state, the aligning device transfers the parallel portion to the position of the mold, thereby preventing the twist in the twisted pair portion from adversely affecting the parallel aligned portion.
Flat cables produced by the above two conventional processes have been utilized as internal wiring materials for various types of electronic equipment including personal computers, and a further increase in demand thereof is expected.
The conventional processes and apparatuses for producing flat cables, however, have the following drawbacks. In the first conventional production process, the distance from the twist ports in the twister to the mold is large. This requires a large space. Further, since the forks are fixed to a belt, it is difficult to vary the spacing between parallel fused portions, that is, the length of the twisted pair portion. Furthermore, many forks, which are disadvantageously expensive, are necessary. An additional problem is such that a large-scale mechanism should be provided for gradually reducing the width of the twisted pair portion and the width of the parallel aligned portion.
On the other hand, the second production process is advantageous in that no remarkably large space is required and, in addition, the necessary number of the aligning device comprising a pair of devices is only one. Since, however, the insulated conductors are arranged in a vertically dispersed state, the insulated conductors become intricate. Therefore, when they are arranged in the horizontal direction, predetermined mutual relationship between the insulated conductors cannot be always provided. This causes the mold to bite the insulated conductors at the time of fusion between adjacent insulated conductors in their insulative layers in the mold, often resulting in damaged insulative layer.